There have been image forming apparatuses that form an image with a color material as a toner decolorized by being heated to a certain temperature or higher temperature.
When a sheet holding a toner image formed from such a decolorable color material becomes a used sheet, the toner image is erased (decolorized) by being heated by means of a decolorizing device to the above-described certain temperature or higher temperature. Efforts are being made nowadays to reduce an environmental load by achieving reuse of sheets with these apparatuses.
A sheet holding an image formed from a decolorable color material is subjected to decolorizing treatment for reuse of the sheet. However, in some cases, an image not having been erased completely may still remain on a sheet after the sheet was subjected to decolorizing treatment. The image before being subjected to the erasure is recognized faintly on a base material. Therefore, if an image is formed again on a side on which this ghost image remains, the visibility of the image printed for the second time and subsequent times is degraded.